This section introduces aspects that may facilitate a better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The concept of carrier aggregation (CA) was introduced in Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) release 10 and now has been widely adopted by global operators.
Carrier aggregation refers to transmitting data on multiple carriers that are contiguously or separately located in a spectrum. In carrier aggregation, each aggregated carrier is referred to as a component carrier (CC). The component carrier may have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers may be aggregated, hence the maximum aggregated bandwidth may be 100 MHz.
A user equipment (UE) supporting carrier aggregation may be configured by a base station, e.g. an evolved Node B (eNodeB or eNB) in an LTE network, with a primary component carrier (PCC) and one or more secondary component carriers (SCCs). There is no definition of which carrier should be used as the PCC. The configuration of the PCC is UE specific and will be determined according to loads on various carriers as well as other relevant parameters. Therefore, different UEs may use different sets of component carriers with different component carriers being configured as the PCCs.
When carrier aggregation is used, there are a number of serving cells, one for each component carrier. The Radio Resource Control (RRC) connection is only handled by a primary serving cell (PCell) served by the PCC. The secondary component carriers serve secondary serving cells (SCells). The SCells may be configured/de-configured, added, removed or modified for the UE via RRC signaling, e.g. RRCConnectionReconfiguration, while the PCell is only changed with a handover procedure. In LTE technology, the terms “PCC” and “PCell” are usually used interchangeably and the terms “SCC” and “SCell” are also used interchangeably.
When a UE is configured with SCells, the configured SCells need to be activated before they can be scheduled for data transmission. The SCells may be activated/deactivated by a base station via Media Access Control (MAC) layer commands, while the PCell configured to the UE is always activated. The base station may activate and deactivate the SCell(s) by sending an Activation/Deactivation MAC control element as described in section 5.13 of 3GPP Technical Specification (TS) 36. 321, V11.5.0. Upon successful reception of the activation command, the UE may be ready to receive assignments on the SCells with a certain time, e.g. 8ms after the activation command was transmitted over the air. So the activation and de-activation procedure may be very fast and controlled totally by the base station.
With the emergence of advanced transport vehicles, especially the development of high-speed trains, it becomes more challenging to provide reliable services for UEs in high-speed movement with efficient utilization of valuable frequency resources.
Taking a high speed train scenario as an example, it has some unique characteristics. For example, the UEs on a high speed train (which will be referred to as HST UEs) move as one group with a high speed, and penetration loss to the train is very high, and thus path loss from the base station to the HST UEs is also high. Normally, if conventional cells along the rail track are re-used to provide coverage for the HST UEs, it is very hard to optimize the coverage and resource/interference management for both the HST UEs and non-HST UEs, which may located close to the rail track but are static or moving more slowly than the HST UEs.
Besides, most operators allocate a higher frequency spectrum for LTE compared with Global System for Mobile Communication (GSM). Therefore, the coverage problem becomes more serious since the path loss and penetration loss are much higher at higher frequency points than lower frequency points.
There are two main solutions for providing coverage for HST UEs, as illustrated in FIG. 1, where each base station is configured with two carriers f1 and f2. FIG. 1(a) illustrates a first solution that may be called a “common” solution, where carriers f1 and f2 are common for HST and non-HST UEs. It also means that when a HST is passing a cell, the non-HST UEs and HST UEs in the cell are sharing the radio resources on carriers f1 and f2, and the base station won't treat them differently with regard to scheduling and/or radio resource management (RRM).
FIG. 1(b) illustrates a second solution that may be called a “dedicated” solution, where carrier f1 is always used for non-HST UEs while carrier f2 is reserved for HST UEs only, no matter whether the HST UEs are present or not.
The common solution as illustrated in FIG. 1(a) cannot achieve good load management as HST UEs are group handed over from a cell to another in a quick speed. The instantaneous load may be very high when taking non-HST UEs into account and therefore UE experience may be deteriorated. In addition, the HST UEs may be subjected to interference from transmissions of neighboring non-HST UEs as both carriers f1 and f2 are simultaneously used by the non-HST UEs. This interference may be very serious mainly because the HST UEs experience much more path loss and no efficient intra-frequency interference handling mechanism is applied. Therefore, the quality of service (QoS) for the HST UEs may not be guaranteed.
The dedicated solution as illustrated in FIG. 1(b) may provide a good QoS to HST UEs without any interference from transmissions of neighboring non-HST UEs because of dedicate use of carrier f2. However, this good QoS is achieved at the cost of low spectrum utilization since the HST service in a cell is limited to a very short time period only when the HST is passing the cell. Therefore, the spectrum cannot be flexibly and efficiently utilized, which causes this solution costly.
In the “dedicated” solution, such a dedicated carrier may be based on a same site/antenna or different sites/antennas, where the latter case will have separate antennas and possibly sites. The dedicated solution with separate sites/antennas may provide optimized and flexible HST coverage more accurately, but this may also increase infrastructure investment and network management/optimization cost, like Physical layer Cell Identity (PCI) handling, and antenna tilting. Sometimes, if a paging channel (PCH) is not optimized between HST cells and non-HST cells, it may cause interference and degrade network performance due to a common reference signal (CRS) collision.
Therefore, the existing solutions are very hard to balance between QoS/interference management and spectrum/cost-efficient HST coverage.